Compressor capacity in refrigerant compressors may be varied, especially in multi-cylinder refrigerant compressors, by providing a two position eccentric cam rotatably mounted on the crankpin. The cam is angularly adjustable in response to reversing the direction of rotation of the crankpin by the crankshaft drive motor. One direction of rotation results in the positioning of the eccentric cam having a more eccentric rotation path to provide compression in a corresponding cylinder, while the opposite direction of rotation results in the position of the eccentric cam having a circular rotational path to provide a different amount of compression or no compression in the cylinder. The use of the two position eccentric cam (i.e., the disengageable eccentric cam) allows the compressor to have variable capacity by effectively removing compression in one of the cylinders for one direction of rotation and allows the compressor to maintain efficiency, while under varying load requirements.
One type of eccentric cam is described in U.S. Pat. No. 4,479,419, hereinafter the '419 Patent. The angular positioning of the cam (i.e., the eccentric cam) on the crankpin is accomplished by providing a pair of drive stops which are angularly spaced on a portion of the crankpin, and a dog provided on the cam. These stops and the dog are angularly positioned with respect to each other such that upon rotation of the crankshaft in one direction a first stop will engage one side of the dog and rotate the cam to a first prescribed angular position on the crankpin to produce one piston stroke length. Conversely, reversing the rotation of the crankshaft disengages the dog from the first stop and causes the cam to rotate and engage the opposite side of the dog to a second stop, which also rotates the cam to a second prescribed angular position on the crankpin to produce another piston stroke length.
A compressor operates by drawing gas into a chamber and compressing the gas during a compression cycle. The end of the compression cycle is when the discharge of gas from the compression chamber ends and drawing of the gas into the chamber begins. Reciprocating compressors having disengageable eccentric structures typically include a piston that compresses gas inside a compression cylinder or chamber. A protrusion on the eccentric cam, called a dog, engages a stop on the crankshaft to facilitate rotation of eccentric cam structure. At the completion of the compression cycle, the compressed gas is discharged from the compression cylinder through a discharge valve in a valve plate at one end of the cylinder. The end of the compression cycle in a reciprocating compressor corresponds approximately to the top dead center position of the piston (i.e., the maximum length the piston extends into the compression cylinder). A volume of gas, commonly referred to as reexpansion gas, is not discharged from the compression cylinder and remains in the clearance space of the cylinder (i.e., the space between the valve plate and piston) at the completion of the compression cycle. The reexpansion gas remaining in the cylinder exerts force on the piston. In reciprocating compressors using a disengageable eccentric cam, a force on the piston from the reexpansion gas transfers through the piston assembly to the disengageable eccentric cam. The eccentric cam is accelerated to a rotational velocity greater than the velocity of the crankpin, which results in a slight disengagement of the disengageable eccentric cam's dog from the stop on the crankpin. The crankpin continues to rotate and the eccentric portion returns to the same velocity as the crankpin. The eventual reengagement of the stop on the crankpin with the dog on the disengageable eccentric cam occurs with substantial momentum and impact, thus producing noise, commonly referred to as chatter. Chatter is a metallic clacking or clicking noise generated by the rapid and forceful reengagement of the stop and dog.
Rotary compressors having disengageable eccentric structures are also susceptible to noise in the form of chatter. Rotary compressors include a roller having an eccentric crank mounted on a crankshaft. A protrusion on the eccentric crank, called a dog, engages a stop on the crankpin to facilitate rotation of the roller structure. The roller compresses gas inside a compression cylinder. At the completion of the compression cycle, the compressed gas is discharged from the compression cylinder through a discharge valve positioned along the inner surface of the cylinder. Like in the reciprocating compressor, a volume of reexpansion gas is not discharged from the compression cylinder and remains in the cylinder at the completion of the compression cycle. The reexpansion gas remaining in the cylinder exerts force on the roller, causing the roller and eccentric crank to accelerate to a rotational velocity greater than the crankpin. The crankpin continues to rotate and the roller and eccentric crank return to the same velocity as the crankshaft. The eventual reengagement of the stop on the crankpin with the dog on the disengageable eccentric crank occurs with substantial momentum and impact, thus producing the chatter.
The problem of chatter is not limited to reciprocating and rotary compressors. Any type of compressor having a disengageable eccentric structure may be susceptible to the problem of chatter.
One attempt to address the problem of disengagement and reengagement of the stop and dog includes placing locking mechanisms for the disengageable eccentric structure on the disengageable eccentric cam. For example, U.S. Pat. No. 6,092,993, herein incorporated by reference, utilizes various latching mechanisms that mechanically hold the disengageable eccentric cam and the crankpin stop together while the crankpin is rotating. However, the latching means requires additional components and/or machining on the rotating crankpin and disengageable eccentric cam to maintain engagement. Also shown in U.S. Pat. No. 6,092,993, is the attempt to address the problem of disengagement and reengagement of the stop and dog using inertial mass to hold disengageable eccentric structure against the crankpin stops. The addition of mass to the eccentric cam shifts the center of gravity of the eccentric cam and acts to provide additional force to maintain engagement while the crankpin is rotating. However, cam inertia is generally ineffective to prevent disengagement, particularly from the force against the disengageable cam caused by reexpansion gas.
What is needed is a method and/or system for reducing noise and chatter in variable capacity compressors with disengageable eccentric structures resulting from reexpansion gas remaining in the cylinder at the completion of the compression cycle.